Some piezoelectric actuators that drive a driven body by means of the vibration of a piezoelectric element are configured to excite a plurality of vibration modes of the piezoelectric element and utilize a combination of these vibration modes to drive the driven body. In order to obtain the desired vibrational path and drive force, such piezoelectric actuators must be designed to maintain the vibration components of each vibration mode in an appropriate manner. As a means of satisfying this requirement, methods have been proposed whereby the drive signal applied to the piezoelectric element is controlled based on a phase difference between the drive signal and a detection signal obtained by detecting the vibrations of the piezoelectric element (see Patent Document 1, for example). The piezoelectric actuator in Patent Document 1 is provided with a detection electrode so that the phase difference between the drive signal applied to the piezoelectric element and the detection signal detected from the detection electrode can be monitored. By adjusting the drive frequency of the drive signal applied to the piezoelectric element such that a prescribed phase difference value is obtained, the vibration components of the different vibration modes of the piezoelectric actuator are controlled appropriately and the driven body is driven in an efficient manner.
[Patent Document 1] Japanese Laid-open Patent Publication No. 2002-291264 (pages 12 and 13, FIGS. 26 and 27)